Equipment for bursting existing pipes and widening the bore of the burst pipe by pressing the debris into the surrounding soil is used for the simple trenchless installation of utility pipes. Unlike the conventional cut-and-cover technique, the use of a trenchless method for the installation of new pipes minimizes disturbances and obstruction to traffic.
West German patent No. 3,800,869 divulges an apparatus for widening existing pipes wherein the head of an impact ramming device (hereinafter referred to as "mole") is provided with a blade and a support surface parallel to the longitudinal center line of said mole. Said support surface parallel to the longitudinal center line of said mole is to concentrate the force of the forward movement at one point for fracturing, said point being the location of the only blade with which said apparatus is provided.
To date, the fracturing of steel lines has posed particular difficulties. Conventional equipment has been fitted with rigidly attached blades which have been mounted on the cone-shaped portion of the mole or a separate cone-shaped body connected with the head of the mole. The diameter of the cone has conventionally been wider than the internal diameter of the pipe to be fractured. If pipes of relatively small diameter are fractured for the installation of new buried pipes, the mole may advance at a speed in excess of the speed at which the cable pulled by a winch moves forward, the cable hence loosing its tension. The guiding control of such a mole may thereupon be lost, the mole escaping from the bore of the existing pipe and damaging utility lines laid close to the existing pipe to be destroyed.
Another known apparatus for widening the bore of an existing pipe comprises a cutting face attached to the mole by a universal joint resisting tensile and compressive forces. The cable by which said apparatus is pulled is fastened to the body carrying the cutting face. Said arrangement cuts the line on two sides at the beginning of the cutting operation but the mole may relatively soon be displaced in the direction of the side offering the least resistance if the mole advances more rapidly than the cable is being pulled, only one side of the pipe thus being cut.
As the mole travels through a pipe section, it turns about its longitudinal axis. To prevent such turning from having a twisting effect on the cable by which the apparatus is being towed, it has to date been necessary to provide for an untwisting device. The blades carried by conventional moles have at least partially moved together with the mole, cuts in the steel pipe to be fractured thus taking a corkscrew-type appearance even if cutters engage with two opposite pipe walls, thereby obstructing the bore widening operation.
The effects described hereinbefore mainly occur when steel pipe is fractured (cut) and widened. Cast iron, asbestos, cement or PVC pipes may be broken more easily. Pipeline contractors must hence dispose of a multiplicity of special moles for steel pipe on the one hand and for pipes made from other material on the other hand, whereby the cost of trenchless utility line installation is substantially increased.